1. Field of the Invention
This invention relates to circuit breakers and, more particularly, to ground fault circuit breakers.
2. Background Information
Ground fault circuit breakers are generally old and well known in the art. Examples of ground fault circuit breakers are disclosed in U.S. Pat. Nos. 5,260,676; and 5,293,522. Ground fault circuit breakers are used to protect people and electrical circuitry from damage due to an overcurrent condition, such as an overload condition, a relatively high level short circuit, or ground fault condition.
In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light industrial applications, overcurrent protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal which is heated and bends in response to a persistent overload condition. The bimetal, in turn, unlatches a spring powered operating mechanism which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
In one circuit breaker, the thermal characteristic responds, for example, to 30 A being drawn in a 15 A circuit. On the other hand, an armature, which is attracted by the sizable magnetic forces generated in a magnetic core by a short circuit also unlatches, or trips, the operating mechanism. As an example, the magnetic type actuation occurs when the hot line conductor becomes directly connected with ground or neutral, thereby bypassing the load.
In many applications, a miniature circuit breaker may provide ground fault protection. Typically, an electronic circuit detects leakage of current to ground and generates a ground fault trip signal. This trip signal energizes a trip solenoid, which unlatches the operating mechanism, often through deflection of the armature of the thermal-magnetic trip device.
Ground fault circuit breakers include both Class A (e.g., ground fault current of about 5 mA for people protection) and equipment protective devices (e.g., ground fault current of about 30 mA).
A common type of ground fault detection circuit is the dormant oscillator detector including first and second sensor coils. The line and neutral conductors of the protected circuit pass through the first sensor coil. The output of this coil is applied through a coupling capacitor to an operational amplifier followed by a window comparator having two reference values. A line-to-ground fault causes the magnitude of the amplified signal to exceed the magnitude of the reference values and, thus, generates a trip signal. At least the neutral conductor of the protected circuit passes through the second sensor coil. A neutral-to-ground fault couples the two detector coils which causes the amplifier to oscillate, thereby resulting in the generation of the trip signal. See, for example, U.S. Pat. Nos. 5,260,676; and 5,293,522, which are incorporated herein by reference.
Often, conventional circuit breakers are positioned to provide up and down movement of the circuit breaker handle as shown in FIG. 1. Normally, the line terminal is located at the top portion and the load terminal is located at the bottom portion of the circuit breaker.
Typically, local codes require that equipment provided by original equipment manufacturers (OEMs) be configured such that the "ON" position of the circuit breaker handle is in the up direction and the "OFF" position of the circuit breaker handle is in the down direction. In that instance, for circuit breakers which do not provide ground fault protection, the line terminal is located at the top and the load terminal is located at the bottom of the circuit breaker.
Often, OEMs choose to wire their equipment from the bottom of the circuit breaker such that the non-ground fault protection circuit breaker provides a "line" terminal (actually its intended load terminal) and a "load" terminal (actually its intended line terminal) which are located at the bottom portion and top portion, respectively, of the breaker. This is easily accomplished by "reverse feeding" the non-ground fault circuit breaker from its intended load terminals as is shown in FIG. 2.
In conventional ground fault circuit breakers, the ground fault detection circuit is powered from the load side of the circuit breaker such that the ground fault detection circuit is not powered after the circuit breaker has detected a ground fault and, thus, has tripped. In this manner, the circuit breaker separable contacts are employed as a cut-off switch to remove power to the ground fault detection circuit and, thus, protect such detection circuit.
In the event that ground fault protection is to be retrofitted into an existing OEM product in which a non-ground fault circuit breaker had previously been "reverse fed" (see FIG. 2), the OEM must redirect the wiring (e.g., within the OEM product as shown in FIG. 3, or by providing an external enclosure for the ground fault circuit breaker). Accordingly, there is room for improvement.